1. Field of the Invention
The present invention generally relates to exploratory tools for use in oil and gas well bores and, more specifically, to acoustic logging devices and methods used to investigate the acoustic coupling between a well casing and a bore hole.
2. Description of the Prior Art
In a typical well completion operation, a casing for pipe is placed in the bore hole and cement is pumped into the annular region defined between the casing and the surrounding bore hole. For various reasons, the cement may fail to completely fill the annulus along the length of the casing resulting in the condition generally known as “channeling.” Additionally, portions of the column of cement may fail to bond to the casing or formations. In such cases, fluids and/or gases from the formations at various depths may migrate along the imperfections in the column of cement. Fluid communication between adjacent porous zones is undesirable.
A number of systems are known in the prior art for obtaining a measure of the quality of a cement bond relative to the casing. For example, U.S. Pat. Nos. 3,291,274; 3,291,248; and 3,292,246 all utilize acoustic principles where an acoustic signal is transmitted between a transmitter and receiver. The amplitude of an early arrival signal at the receiver is measured as a determination of the quality of the bond of the cement to the casing (this early arrival usually is the casing signal since the acoustic energy under average conditions generally travels faster in the casing than in the surrounding cement or formation). If a good bond exists, the casing signal can be expected to be attenuated because of the energy dissipated from the casing to the cement and surrounding formations. On the other hand, if no bond or a relatively poor bond exists, the casing signal can be expected to be relatively unattenuated.
Other techniques are also known for determining the quality of cement in the annulus between the casing and formation. For example, in U.S. Pat. No. 3,401,773, the amplitude of a reverberated casing signal is captured and recorded and, additionally, the total energy of a selected later portion of the sonic signal is obtained by integration to provide a second indication of the quality of the cement bond.
The previously described devices supported the well logging tool on an electric line so that the information gathered down hole could be sent to the surface continuously. All of the information is sent up the electric line from the tool located down hole to surface equipment where the information is electronically removed from the line and analyzed in a method allowing the signals to be displayed in real time relating to a given depth in the bore hole.
It would be desirable to provide a method of running a cement bond log on a “slickline” (a supporting wire or cable with no electrical conductor). The absence of an electrical conductor, however, necessitates that data be gathered and stored in a memory module located in the tool itself. The data could be collected and stored while running the log and later downloaded and processed at the well surface. To this point, such a technique has proved to be impractical primarily due to the large volumes of memory storage required to store the data in question.
Another object of the present invention is to obtain such information using a slickline well apparatus.
The present invention has as an objective to provide new and improved devices and methods for determining, by acoustic velocity techniques, the quality of the cement behind a casing.
Another objective of the invention is to provide a device and method for determining the depth of a “stuck pipe” in a pipe recovery process in a well bore using a combination of slickline (memory logging) and improved sonic signal acoustic amplitude techniques.